Page 132 - Digital Electronics by harish
P. 132
9. Program Counter (PC)
The Program Counter (PC) is a 16-bit register. It is used to point the address
of the next instruction to be fetched from the memory. When one instruction is
fetched from memory, PC is automatically incremented to point out the next
instruction.
10. Incrementer / Decrementer
This unit is used to increment or decrement the contents of the 16-bit
registers.
11. Timing and Control unit
This unit synchronizes all the microprocessor operations with the clock and
generates the control signals necessary for communication between
microprocessor and peripherals. The internal clock generator is available in this
unit.This unit has the micro programs for all the instructions to carry out the
micro steps required in completing the instructions. This unit receives signals
from the Instruction decoder and Machine cycle encoding unit and generates
control signals according to the micro-program for the instruction.
12. Interrupt control
There are five hardware interrupts available in 8085 Microprocessor namely
TRAP, RST 7.5, RST 6.5, RST 5.5 and INTR for interfacing the peripherals
with the microprocessor. These interrupts are handled by the Interrupt control
unit. signal is generated by the Interrupt control unit as an
acknowledgement for an interrupting device. If two or more interrupts occur at
the same time, service is given according to the priority basis.
13. Serial I/O control
Serial data is transmitted to the peripherals through SOD pin and received
through the SID pin. The SOD and SID pins are handled by the Serial I/O
control unit using the SIM and RIM instructions.
14. Address buffer and Address / Data buffer
The Address buffer is an 8-bit unidirectional buffer from which the higher
order address bits A – A leaves the microprocessor to the memory and
15
8
peripherals. The Address / Data buffer is an 8-bit bidirectional buffer used for
sending the lower order address bits A – A and sending and receiving the data
0
7
bits D – D to the memory and peripherals.
0
7
132
